Field of the Invention
The present invention relates to a technique of correcting degradation in image quality in the peripheral region of a screen using a stabilization mechanism which stabilizes an image blur.
Description of the Related Art
There has conventionally been known an image capturing apparatus having an image stabilization function of detecting the shake of the image capturing apparatus and then driving an image stabilization lens to stabilize an image blur caused by this shake. An angular velocity sensor is generally used to detect the shake of an image stabilization apparatus. The angular velocity sensor vibrates a vibration material such as a piezoelectric element at a predetermined frequency and outputs a voltage corresponding to the Coriolis force generated by a rotational movement component as angular velocity information. The image stabilization apparatus obtains the amount and the direction of the shake by integrating the acquired angular velocity information and outputs a stabilization position control signal which drives the stabilization lens to cancel the image blur. When driving the stabilization lens, feedback control is performed in which the current position of the stabilization lens is fed back to the image stabilization apparatus as the position signal of the stabilization lens and the image stabilization apparatus outputs a stabilization position control signal corresponding to the position signal of the stabilization lens.
In general, image data obtained by the image capturing apparatus tends to become darker and have a lower resolution in the periphery including four corners than in the center. In particular, a phenomenon in which the light amount becomes smaller in the periphery than in the center is referred to as shading and a phenomenon in which the resolution becomes lower on one side than in the center is referred to as an one-sided blur, respectively. As the rate of decrease in the light amount, a variation in luminance, or a decrease in the resolution at the four corners is larger in the peripheral region than in the center, the quality of the image data deteriorates.
The degree of decrease in the light amount or the resolution is a characteristic that the lens originally has. Therefore, it can be said that the variation in luminance or the resolution at the four corners is caused by that characteristic. The degree of decrease in this light amount or resolution also varies in accordance with a focal length, that is, a zoom ratio. An example of the decrease in the resolution will be described below.
Each of FIGS. 6A and 6B is a graph of showing, as an example, a resolution with the abscissa plotting a distance from an optical center and the ordinate plotting the rate of the resolution (the resolution in the optical center is set to 1). As shown in FIG. 6A, the resolution on one side in the periphery of a lens decreases the most on the telephoto-side and hardly decreases on the wide angle side. Furthermore, as shown in FIG. 6B, when the resolution in the periphery decreases significantly, a variation in the resolution is increased by, for example, the mechanical error or play of a lens barrel which bundles respective lens groups.
A method of reducing a decrease in a light amount or the decrease in the resolution by using the arrangement of the image stabilization function is available. A case in which a shift lens is used as an image stabilization member in the image stabilization mechanism will be described here.
It is possible to keep a decrease in a light amount (or a resolution) at four corners within a predetermined level in the entire zoom range by adjusting the center position of a shift lens at a focal length where the decrease in the light amount (or the resolution) is the largest (Japanese Patent Laid-Open No. 2007-57981). Such adjustment of the center position of the shift lens will be referred to as “shading correction” (or “resolution correction”) hereinafter. The center position adjusted by this shading correction (or this resolution correction) may differ from the center of a movable range where the shift lens can be moved mechanically.
With the conventional technique disclosed in Japanese Patent Laid-Open No. 2007-57981, however, when performing shading correction or resolution correction using the shift lens, a variation in the angle of view is noticeable at the time of a zoom operation if a correction amount varies greatly between zoom operations.